The loose ODB silently dropped every write on device: utimes() returned 0
unconditionally, so libgit2's freshen probe (git_futils_touch -> p_utimes)
always reported "object exists" and git_odb_write skipped the write entirely
-- blobs/trees/commits never hit disk and write_tree failed with "invalid
object specified". Gate utimes on file existence (present -> 0, absent ->
ENOENT). Also add a remove-then-rename p_rename (FATFS f_rename can't replace
an existing target, no hardlinks), with posix.c's original scoped out in the
component CMakeLists, plus symlink/gai_strerror link stubs git_push pulls in.
p_rename was verified on hardware (cleared the rename error); the utimes fix
is diagnosed from the failure + libgit2 source and build-verified, on-device
confirmation still pending.
8d80168bda